You will have to configure the fridge so that it cannot back-feed from the van battery when the tug is disconnected.
Check above Fellas - already covered. With a modern 3 way fridge, it has an ignition sensor wire to cover this. Thinking outside the square, Interupt the main feed to the fridge with a N.O. relay which is energised by a feed from the van side/tail-lamp circuit. Will only operate the fridge when the sidelamps are on and will turn off the fridge when the trailer plug is disconnected or the tug lights are turned off. Simple and cheap.
Derb. 2017 Colorado LTZ 4 x 4 Auto. 2014 Southern Star 720 and 1980 Zephette 340. 2 x 100 a/h AGM. 3 x 150w solar. Ctek D250S-Dual controller/MXS15 charger.172L fresh/grey. Diesel heat. RMD8555 fridge.